1. Field of the Invention
The invention relates to a device for transmitting electrical signals or energy between a plurality of units that are movable relative to each other.
For the sake of clarity of presentation, no distinction will be made in this document between a transmission between units that are movable relative to each other, and between one fixed unit and units movable relative thereto, because this is only a matter of positional relationship and does not affect the manner of operation of the invention. Similarly, no distinction will be made between a transmission of signals and of energy, because the mechanisms of operation are the same in both cases.
2. Description of the Prior Art
In the case of linearly movable units such as crane and conveyor systems, and also computer tomographs, it is necessary to transmit electrical signals or energy between units that are movable relative to each other. A device suitable for this is described in the German laid open Patent Application DE 44 12 958 A1. Here a signal to be transmitted is fed into a strip conductor line of the first unit that is disposed along a path of movement of the units that are movable relative to each other. The signal is tapped off from the second unit by means of a capacitive or inductive coupling. An improved device for transmission, as described for example in WO 98/29919, is based on a specific conductor structure that also possesses filter characteristics. Extremely wideband transmission systems operating in a range from a few MHz up to Ghz may be obtained with structures of this kind. In the following expositions, the term conductor structures relates to all conceivable configurations of conductor structures which are suitable for carrying electrical signals. The signals are decoupled in the near field of the conductor structure. In an ideal case, a decoupling of signals should occur exclusively within range of the second unit. Further emission of signals in other ranges of the conductor structure is not desirable, as distinct from the case of known leakage lines, because the wide band signals can lead to interference in other instrument parts or instruments.
The principles of designing and dimensioning leakage lines, such as described in the U.S. Pat. No. 5,936,203, cannot be applied to conductor structures of this kind. Leakage lines are specifically designed to radiate a certain portion of the carried high frequency energy outwards along the entire length. This is exactly what is to be avoided here.
Technically similar to a non-contacting decoupling of signals is also a contacting decoupling of signals. However, a non-contacting decoupling is usually preferred, because it is more reliable and free from needing maintenance.
The conductor structures described here may be optionally designed to be contacting or also non-contacting. In this case, of course, adaptations are possible according to the purpose of the transmission. Thus, a conductor structure for contacting transmission may have a particularly well conducting surface, for example with a silver coating. Contrary to this, a conductor structure for non-contacting transmission may be provided with a lacquer coat on the surface as corrosion protection. However, in these cases the basic principles for designing the conductor structures are identical. A special design of a contacting transmission means is described in the U.S. Pat. No. 5,208,581. Here an unbalanced conductor system is also described. Although here the configuration is symmetrical, the conductor system is fed with an unbalanced signal. The signal flow from the transmitter to the receiver is effected via a central conductor, and is returned partly via one or both outer conductors, or even via the computer tomograph system itself. Here the instrument itself is a reference surface. Here the configuration of the reference surface is not designed to be unequivocally symmetrical. Because of the unbalanced signals that have a not unequivocally defined signal path, and the undefined reference surface, this system radiates high HF power. Already with data rates of 50 Mbaud, current EMC Standards can no longer be satisfied without additional costly screening.
The conductor arrangements used here for transmission are usually constructed to be strip conductor lines or conductor structures by means of double-sided printed circuit boards. Glass-fiber reinforced plastics usually serve as a support and a dielectric. This support is provided on one side with a continuous conductor surface as electrical reference surface or mass, and on the other side with a strip-shaped conductor or the conductor structure.
One of the most difficult technical problems with transmission systems of this kind is the achievement of a high resistance to interference and also a low emission of radiation emission. Now, in order to achieve a signal transmission with particularly low interference, for example two parallel conductors or conductor structures are symmetrically supplied with a differential signal. This makes the far field approximately equal to zero, at least for conductor spacings that are smaller than the wavelength. Thus, only extremely low energy is radiated. In the opposite case, in the event of undesired coupling-in of electromagnetic waves from the outside, the same signal is produced in both conductors. This may now be filtered off by a receiving circuit having a high common mode rejection. Symmetry of the entire arrangement is essential for a high resistance to interference.
Normally, the signal level of the transmitter cannot be increased arbitrarily in order to increase the resistance to interference. Despite high symmetry, low radiation will always occur. The radiation decreases with higher symmetry, and the signal levels can be increased further.
At large bandwidths or data rates within the range from a few 100 MHz up to several GHz, attenuations or distortions of the signals, which are not negligible, will occur. Thus, attenuations of the order of magnitude of 10 dB per meter were measured for usual conductor materials and a frequency of 1 GHz. For great lengths this leads to unacceptable attenuations. Furthermore, there is an increased risk of non-symmetries, because the dielectric cannot be fabricated to be as symmetrical or homogeneous as desired.
A solution that avoids these problems from the outset is set out in the U.S. Pat. No. 5,287,117. With this, the conductor arrangement is replaced by a plurality of small antenna segments. These may be manufactured with high quality materials on printed circuit boards of small area. Feeding via long distances can be effected with high quality coaxial cables having highly screening characteristics and low attenuation. However, here too, owing to the large number of antenna segments, large requirements of material and, in particular, a large outlay of assembly work result, which leads to high fabrication costs.